Resolve issues IRQ handling, IPC, and fpage

[jserv]

IRQ handling:

• Add memory clobbers to cpsid/cpsie to prevent reordering
• Fix irq_number() operand order (was reading garbage)
• Add isb after msr control for pipeline synchronization
• Restore r4-r11 in schedule_in_irq() when no context switch
• Make __irq_saved_regs volatile to prevent optimization issues
• Use explicit unrolled copy in __irq_save to prevent loop issues

IPC:

• Add MR bounds check in INACTIVE non-start path (security)
• Return error for send to T_INACTIVE without timeout (prevents hang)
• Return error for send to non-existent thread

Fpage management:

• Fix split_fpage() bounds validation (<= base, >= end)
• Add NULL checks for as/fpage parameters
• Fix shift UB: use ((memptr_t)1 << shift) throughout
• Add overflow check for end = base + size
• Add free_fpage_chain() helper for proper cleanup
• Fix partial allocation leaks in assign_fpages_ext()
• Add cycle guard in unmap_fpage() with CONFIG_MAX_FPAGES limit

Memory/Thread subsystem:

• Add error handling for assign_fpages_ext() failures
• Add panic on KIP fpage creation failure
• Check map_fpage() return values in map_area()